We write code, and we write tests. When you write tests, you're thinking about both the expected usage of the thing you're testing - and you're also thinking about the internal implementation of that thing, writing your test to try exposing bad behaviors.
Whenever you change the implementation of the thing, you're adding new lines of code, and sometimes it's difficult to be sure that your test actually exercises all the code in the thing. One thing you can do is set a breakpoint, and step through painstakingly. Sometimes you have to. But I'm looking for a faster, better way to ensure all the code has been tested.
Imagine setting a breakpoint and stepping through the code, but every line that was run gets highlighted and stays highlighted after it was run. So when the program finishes, you can easily look at the code and identify lines which were never run during your test.
Is there some kind of tool, extension, add-in, or something out there, that will let you run a program or test, and identify which lines were executed and which were not? This is useful for improving the quality of tests.
I am using Visual Studio and Xamarin Studio, but even if a tool like this exists for different languages and different IDE's, having that information will be helpful to find analogous answers on the IDE's and languages that I personally care about.
As paddy responded in a comment (not sure why it wasn't an answer), the term you're looking for is coverage. It's a pretty critical tool to pair with unit testing obviously because if some code isn't covered by a test and never runs, you'd want to know so you can more completely test. Of course, the pitfall is that knowing THAT a line of code was touched doesn't automatically tell you HOW the line was touched - you can have 100% test coverage without covering 100% of use cases of a particular LOC - maybe you have an inline conditional where one bit never gets hit, just as an example.
Coverage is also useful to know even outside of testing. By performing a coverage analysis on code in production, you can get a feel for what aspects of your codebase are most critical in real use and where you need to focus on bugfixing, testing, and robustness. Similarly, if there are areas of your codebase that rarely or never get hit in a statistically significant period of production uptime, maybe that piece of the codebase can or should get trimmed out.
dotCover and dotTrace seem like they'll probably put you on the right path. Coming from use in Python, I know I used to use Django-Nose, which comes with coverage testing and reporting integrated. There's coverage.py for standalone analysis, and of course these aren't the only tools in the ecosystem, just the ones I used.